This invention relates to a method for electrically determining the existence of insulation deterioration of a power cable system, and specifically to a novel and unobvious method for diagnosing the insulation deterioration caused by the absorption of moisture into the solid insulation material.
Insulation deterioration of a power cable insulated with rubbers or plastics, for example, a polyethylene covered overhead power cable (hereinafter referred as CV cable) is evidently induced by water trees. Therefore, it is important to know of the generation of the water trees to prevent dielectric breakdown caused by insulation deterioration in the dielectric characteristics of the CV cable.
In the prior art, two methods for detecting the presence of water trees are known. A first method consists of sampling a portion of the insulation from an inactive cable in which a dielectric breakdown has occurred, boiling or dying the sample, and visually inspecting the sample directly by eye or with an optical microscope. The second method consists of applying a DC voltage to an inactive cable, measuring the charging current stored in the cable after application of the voltage, and estimating the generation of water trees in accordance with the result measured.
According to these methods, the water trees are detected in an inactive cable under no load conditions since dielectric breakdown of a cable cannot be detected reliably beforehand in an actually working power line system. Other known methods include testing a dielectric strength by applying a high AC or DC voltage to a power cable, and a method for measuring DC leakage current by applying a DC voltage thereto. The former, which is of a destructive method, may induce a dielectric breakdown by the test itself depending upon the degree of prior insulation deterioration. On the other hand, even though breakdown does not occur, it is possible that the test itself may cause the insulation characteristics to further deteriorate. The latter method is generally used to measure the mean insulation resistance over the entire length of the cable as indicated by the magnitude of the leakage current, and to observe pulse current generated intermittently in the defective portion thereof (called Kick Phenomenon). A disadvantage, however, is that the method necessitates the use of a high DC voltage generator and that even nondefective cable may be subject to possible damage by a steep and unusual voltage which is generated when a flash-over or a dielectric breakdown occurs. Another problem with such methods is due to differences between the AC voltage present during normal cable use and the DC voltage used for the test, including a significant difference in the electric field produced so that the test is not regarded to be a characteristic check under the electric field in accordance with the application of a commercial AC voltage.
Recent research teaches that the magnitude of leakage current is greatly effected depending upon the polarity of the DC voltage when the water tree arises in the insulation and the electrodes are arranged asymmetrically. This phenomenon results in the false estimation of insulation deterioration due to the change in magnitude of the current.
In consideration of the above, a method is proposed for checking the cable which comprises applying an AC voltage to a cable undergoing testing, detecting the DC current component of the current through the grounding line, analyzing the polarity, magnitude, and time lapse characteristics of the DC current, and finally detecting the existence, magnitude and growth direction of the water tree in the cable insulation.
Observing water trees in one hundred specimens of 6 KV class-CV cables, having effective lengths of 10 m and wherein the actual working voltage of 3.8 KV for the CV cable was applied to the conductor, the DC current component was detected from the outer shielding of the cable, and AC breakdown was carried out in each specimen. The results are shown in FIGS. 1 to 3.
FIG. 1 shows the relationships between the percentage volume of the water trees observed in the cable insulation and the DC current component through the grounding line per lm.sup.3 expressed as microamperes per cubic meter (.mu.A/m.sup.3). The component is measured in units of .mu.A/m.sup.3 to normalize measurements for cable specimens having different conductor sizes. In FIG. 1, curve A indicates the water tree generated from the conductor shield of the cable, and the curve B indicates the water tree generated from the insulation shield of the cable. It can be seen from these curves that the larger the DC component of the cable, the more the water tree occupies the remarkable portion of the insulation in its volume.
FIG. 2 shows the relationships between the occupied volume of the water tree in the cable insulation and the maximum lengths of the water tree from conductor shields and the water tree from insulation shield. This indicates that the larger the volume the water trees occupied in the cable insulation, the larger the maximum length of the water tree becomes.
FIG. 3 shows the relationships between the AC breakdown voltage of the cable and absolute value of the DC current component expressed in .mu.A/m.sup.3. From FIG. 3, it is seen that the larger the magnitude of the DC current component of the cable, the more the AC breakdown voltage is lowered.
From the above, we can conclude the following:
(1) The DC current component is generated by the existence of the water tree. When the water tree from conductor shield is generated, the polarity of the DC current component is negative, and when the water tree from insulation shield is generated, the polarity of the DC current component is positive. PA0 (2) The larger the magnitude of the DC current component, the larger the volume occupied in the cable insulation by the water tree. PA0 (3) The larger the magnitude of the DC component, the longer is the water tree generated.
FIGS. 4 and 5 show examples for preventing dielectric breakdown of a working cable by detecting the existence, magnitude, and growth direction of a water tree considering the polarity and magnitude of the DC current component. FIG. 4 illustrates an example of a CV cable with three cores, and FIG. 5 three CV cables each having a single core.
In FIGS. 4 and 5, a power source transformer 1, high voltage bus lines 2, a grounding transformer 3, CV cables to be diagnosed 4, 4', grounding line 5, 5' connected to the metal shield layer of the cable, and an apparatus for measuring the DC current component are shown.
The apparatus 6 comprises a filter circuit, amplifying circuits, processing circuits, and displaying circuits. The above systems can detect the existence, magnitude, and direction of growth of a water tree by observing the DC component of the current through grounding lines 5 or 5' which are connected to the metal shield layer of the CV cable 4 or 4' to which the AC voltage is applied from bus lines 2. Therefore, the apparatus can be useful for the detection of the water tree not only in a working cable under live conditions, but in an inactive or removed cable.
It was found however, that the above described apparatus was not suitable for use with all cable systems. That is, high voltage cables used in power line systems have plural grounding points at their terminal ends or along the insulation shield thereof. Further, some power cables have an outer shielding layer which is in direct contact with earth ground. In such ground cables, interrupting the grounding means for inserting the apparatus thereinto becomes difficult necessitating the use of another measurement procedure.
As electric power is of critical importance, the sudden interruption of electricity caused by dielectric breakdown of a power cable poses a community problem. According to the present invention, the prior art disadvantages and inconveniences in the diagnosis of insulation deterioration for power cables are substantially eliminated. Therefore, the present invention provides a method for diagnosing power cables to predict dielectric breakdown thereof, and to provide additional warning and time for the replacement or repair of the power line system. As explained above, the present invention is useful not only to the power industry but to our whole society as well.